1. Field of the Invention
The present invention relates to a method for changing radio channels in a radio communication system, particularly in a mobile radio system, in which an existing duplex radio link exhibits both a first radio channel for transmitting communication information via an air interface and a second radio channel for transmitting communication information in the opposite direction to the first radio channel via the air interface.
2. Description of the Prior Art
It is known to use physical channels in radio communication systems for transmitting communication information. By using these physical channels, the communication information, especially speech data or computer data, is transmitted from a first radio station to a second radio station via an air interface. In the case of duplex radio links, communication information is also transmitted in the opposite direction via the same air interface; i.e. in the direction from the second radio station to the first radio station.
Parameters of the physical channels are, for example, a certain timeslot in a TDMA (Time Division Multiple Access) radio communication system, a certain carrier frequency which is used in the transmission of the communication information in an FDMA (Frequency Division Multiple Access) radio communication system, and a certain code via which the communication information is coded for the radio transmission in a CDMA (Code Division Multiple Access) radio communication system. Combinations of the known multiple access methods TDMA, FDMA and CDMA are possible. In a combined TDMA/FDMA radio communication system, for example, a physical radio channel is defined by its timeslot and either its radio frequency or carrier frequency, respectively.
In known mobile radio systems, especially in the GSM (Global System for Mobile Telecommunication), the radio channels via which communication information can be transmitted between a certain base station and a certain mobile station are issued centrally by a coordination unit. The coordination unit selects the individual controls of the base stations operated in the GSM and assigns to them the radio channels.
However, radio communication systems are also known which operate in a so-called uncoordinated mode. In such systems, the radio channels are not issued centrally for the entire system but, instead, the radio stations involved in a radio link select their own radio channels from an existing pool of available radio channels. An example of a radio station operating in uncoordinated mode is the mobile station of a mobile radio system according to the DECT standard.
As already mentioned above, duplex radio links are set up for transmitting bidirectional data, voice information or communication information of other services via an air interface in known radio communication systems, especially in a GSM, a system according to the DECT standard or in a future UMTS (Universal Mobile Telecommunication System). In particular, the TDD (Time Division Duplex) method is known in which a first radio channel and a second radio channel of the same duplex radio link use different timeslots of the same carrier frequency. In this arrangement, the first radio channel represents the radio link in one direction and the second radio channel represents the radio link in the opposite direction. Furthermore, the FDD (Frequency Division Duplex) method is known in which the first radio channel of the duplex radio link uses a first radio frequency and the second radio channel of the duplex radio link uses a second, different radio frequency.
In the duplex radio links of known radio communication systems, there is a fixed association between each pair of the available physical radio channels. In a DECT mobile radio system, for example, in each case 24 timeslots of 10 carrier frequencies are available for the physical radio channels. In this arrangement, the first 12 timeslots of the carrier frequencies are in each case reserved for the downlink radio link between a base station and a mobile station. Timeslots 13 to 24 of the carrier frequencies are reserved for the uplink radio links in the opposite direction from the mobile station to the base station. However, the fixed association in the case of each pair of radio channels does not allow arbitrary combinations of a downlink radio channel and an uplink radio channel in a duplex radio link but decides that each downlink radio channel is permanently associated with a certain uplink radio channel. In the case of the DECT system, the two associated radio channels in each case use the same carrier frequency, the downlink radio channel and the uplink radio channel also using the timeslot which has the same ordinal number in the available 12 timeslots which are allocated to the downlink radio channels and the uplink radio channels. For example, the first timeslot of a carrier frequency is thus permanently joined to timeslot 13, the second timeslot is permanently joined to timeslot 14, and so on.
In GSM, too, there is a fixed association between the uplink radio channel and the downlink radio channel of a duplex radio link. In distinction from the DECT systems, the associated radio channels use different radio frequencies. However, a pair of associated radio channels is in each case allocated to the timeslot having the same ordinal number in the two radio frequencies used. Thus, for example, in each case timeslot 1 of two radio frequencies or timeslot 2 of two radio frequencies and so on is used by a duplex radio link.
During the operation of existing duplex radio links, disturbances which can lead to a loss of the communication information transmitted can occur, for example, due to interference, multi-path propagation and/or fading of communication signals. To prevent further disturbances or to eliminate the disturbance of the duplex radio link, it is typical to change the physical radio channels of the duplex radio link involving both the radio channel for one direction of transmission and the radio channel for the opposite direction.
This situation is explained in greater detail in FIG. 1 of the attached drawing. FIG. 1 shows a total of eight physical radio channels which exist in a radio communication system with TDMA multiple access components. The individual physical radio channels are defined by the common carrier frequency f and by in each case one of the eight timeslots t1, t2 . . . t8. Before a disturbance occurs, an existing duplex radio link uses radio channel Da with the frequency/timeslot combination f/t1 in the downlink direction and radio channel Ua with the frequency/timeslot combination f/t5 in the uplink direction. Then a disturbance occurs, for example in the downlink radio channel Da which is subjected by the mobile station due to an unacceptably high bit error rate. The base station then determines a new available downlink radio channel by accessing a list of the available radio channels which carries an entry for each radio channel whether the radio channel is also disturbed or otherwise used.
In this manner, the mobile station determines the radio channel having the frequency/timeslot combination f/t3 as a possible new downlink radio channel Dn and informs the base station that a change to this radio channel will be initiated and, respectively, that the base station should transmit on radio channel Dn after the change. According to defined protocols, both the change of the downlink radio channel from radio channel Da to radio channel Dn and the change of the uplink radio channel from radio channel Ua to radio channel Un is then performed. The vertical line between the radio channels having timeslots t1 to t4 and between the radio channels having timeslots t5 to t8 indicates that the radio communication system under consideration is a system in which the first four radio channels of the carrier frequency f are exclusively used for downlink connections and the second four radio channels are exclusively used for uplink radio links.
From EP 0 399 612 A2, a method and a system for data transmission via a TDD channel is known in which at least one bidirectional (duplex) voice channel formed of two non-adjacent timeslots is provided for a data transmission between a first radio transceiver station and a second radio transceiver station. In this arrangement, when two or more duplex voice channels are used—if more data packets are to be transmitted in one direction of transmission than in the opposite direction, timeslots of the duplex voice channels allocated to the opposite direction and unused are released and made available for the transmission of the data packets in the direction of transmission.
The present invention is, therefore directed to a method for changing a radio channel of the type initially mentioned, in which the frequency spectrum of the available radio channels can be utilized as well as possible and in which the change can be performed in the shortest possible time.